The present invention relates to a system for a four-wheel drive vehicle for automatically changing the transmission system from four-wheel drive to two-wheel drive.
During driving of a four-wheel drive vehicle, tire scraping occurs because of slight differences in effective wheel radii resulting from inevitable differences in tire inflation, tread wear or variation in loading. In addition, when the vehicle negotiates corners, braking phenomenon called "tight corner braking" may occur during sharp cornering. This is caused by the front wheels running through an arc of greater radius than that of the rear wheels and therefore tending to rotate faster than the rear wheels. This will result in increase of tire wear and fuel consumption and decrease of driveability.
In order to eliminate such disadvantages, a four-wheel drive vehicle provided with a friction clutch system having a variable transmission torque has been proposed. Generally the transmission torque is controlled in dependency on the steering angle, allowing slipping to occur in the friction clutch to prevent the tight corner braking. However, the steering angle at which the tight corner braking phenomenon occurs is not a constant value, but varies with the vehicle speed. If the vehicle speed is low, the tight corner braking occurs at a small steering angle. Therefore, the conventional system in which the changing of four-wheel drive to two-wheel drive is carried out at a constant steering angle can not prevent the tight corner braking at low vehicle speed.